1. Field of the Invention
This invention relates to bacterial diseases of humans and other mammals. In particular, the invention provides a novel signaling factor involved in regulating bacterial growth and pathogenesis, analogs and derivatives of the signaling factor, and methods for controlling bacterial growth and pathogenesis through use of such analogs and derivatives.
2. Background of the Invention
Intercellular cooperation confers a considerable advantage on multicellular organisms that was thought to be unavailable to unicellular organisms such as prokaryotes. Research in the last twenty years has revealed, however, that prokaryotes can communicate with each other in a way that modulates gene expression, and thereby can reap benefits that would otherwise be exclusive to eukaryotes. This ability was discovered in luminous marine bacteria such as Vibrio fischeri and Vibrio harveyi, which activate the expression of genes involved in light production only when their population density exceeds a critical value. This phenomenon, known as quorum-sensing, is now recognized as a general mechanism for gene regulation in many Gram-negative bacteria, and it allows them to perform in unison such activities as bioluminescence, swarming, biofilm formation, production of proteolytic enzymes, synthesis of antibiotics, development of genetic competence, plasmid conjugal transfer, and sporulation.
Quorum-sensing bacteria fall into two classes, depending on how many density-sensing systems they have. Both classes synthesize, release, and respond to signaling molecules called autoinducers to control gene expression as a function of cell density. Bacteria in the larger class use acyl-homoserine lactone signals in a single density-sensing system, with one gene that encodes an autoinducer synthase, and another that encodes a transcriptional activator protein that mediates response to the autoinducer. These genes are homologous to luxI and luxR of V. fischeri, respectively (Bassler and Silverman, in Two component Signal Transduction, Hoch et al., eds, Am. Soc. Microbiol. Washington D.C., pp 431-435, 1995).
Many bacteria that use the autoinducer-1 signaling factor associate with higher organisms, i.e., plants and animals, at some point during their lifecycles. For example, Pseudomonas aeruginosa, an opportunistic pathogen in humans with cystic fibrosis, regulates various virulence determinants with autoinducer-1. Other examples of autoinducer-1-producing bacteria include Erwinia carotovora, Pseudomonas aureofaciens, Yersinia enterocolitica, Vibrio harveyi, and Agrobacterium tumefaciens. E. carotovora infects certain plants and creates enzymes that degrade the plant's cell walls, resulting in what is called “soft rot disease.” Yersinia enterocolitica causes gastrointestinal disease in humans and reportedly produces an autoinducer. P. aureofaciens synthesizes antibiotics under autoinducer control that block fungus growth in the roots.
Bacteria of the other class, exemplified by V. harveyi, have not one but two independent density-sensing systems. V. harveyi apparently uses the more species-specific Signaling System 1 for intra-species communication, and the less species-selective Signaling System 2 for inter-species communication (Bassler et al., J. Bacteriol. 179: 4043-4045, 1997). Each system comprises a sensor-autoinducer pair; Signaling System 1 uses Sensor 1 and autoinducer-1 (AI-1), while Signaling System 2 uses Sensor 2 and autoinducer-2 (AI-2)(Bassler et al., Mol. Microbiol. 13: 273-286, 1994). While autoinducer-1 is N-(3-hydroxy butanoyl)-L-homoserine lactone (HSL)(see Bassler et al., Mol. Microbiol. 9: 773-786, 1993), the structure of autoinducer-2 has not been established, nor have the gene(s) involved in its biosynthesis been identified.
Recent research indicates that quorum-sensing takes place not only among luminous marine bacteria, but also among pathogenic bacteria, where it regulates the production of virulence factors that are critical factors in bacterial pathogenesis. Thus, it would be an advance in the art to identify and characterize compounds with autoinducer-2 activity and the genes encoding the proteins required for production of the naturally-occurring autoinducer-2. Such an advance would provide a way to identify compounds useful for controlling pathogenic bacteria, a way to augment traditional antibiotic treatments, and a new target for the development of new antimicrobial agents.